Germany has spent $200 billion over the past two decades to promote cleaner sources of electricity. That enormous investment is now having an unexpected impact — consumers are now actually paid to use power on occasion, as was the case over the weekend. Power prices plunged below zero for much of Sunday and the early hours of Christmas Day on the EPEX Spot, a large European power trading exchange, the result of low demand, unseasonably warm weather and strong breezes that provided an abundance of wind power on the grid.

Such “negative prices” are not the norm in Germany, but they are far from rare, thanks to the country’s effort to encourage investment in greener forms of power generation. Prices for electricity in Germany have dipped below zero — meaning customers are being paid to consume power — more than 100 times this year alone, according to EPEX Spot.

What causes negative prices? Basically, when the supply of power outstrips demand for it. The energy supplies that Germany depends on are less predictable than they used to be. Wind power, in particular, is highly dependent on changes in weather patterns. Giant spinning turbines produce, on average, about 12% of Germany’s power, but on windy days, they can generate several times that amount. At the same time, other mainstays of the electricity supply, especially some coal and nuclear power plants, are unable to dial back quickly enough, leading to negative prices on electricity trading markets.